This invention relates to a thermal shield for protecting relatively high-temperature components. It is particularly, but not exclusively, related to a shield for the components of a vehicular internal combustion engine exhaust system.
A vehicle's exhaust system includes the engine's exhaust manifold, a catalytic converter, a muffler, and piping connecting the exhaust manifold to the converter, the converter to the muffler, and the muffler to ambient. For proper operation, the catalytic converter must reach a "light-off" temperature. The catalytic converter operates most effectively above the light-off temperature. Above some maximum operating temperature, however, the catalytic converter may be permanently damaged. The light-off temperature and the operating temperature range vary in accordance with the design of the system, particularly the catalytic converter design. The light-off temperature may typically be on the order of 300.degree. to 400.degree. C. (600.degree. F. to 800.degree. F.); the maximum operating temperature may range from about 600.degree. to 1,200.degree. C. (1,100.degree. F. to 2,200.degree. F.). Causing the converter to reach its operating temperature quickly, without causing it to overheat has been the subject of considerable effort, as reflected for example in Benson et al., U.S. Pat. No. 5,477,676, Moore, III, U.S. Pat. No. 5,419,127, Clegg, U.S. Pat. No. 5,390,494, and Bainbridge, U.S. Pat. No. 5,163,289, the disclosures of which are incorporated herein by reference. These systems involve insulating the piping, the catalytic converter, or other exhaust system components, or heating the catalytic converter with an outside heat source.
The necessarily high temperature of the exhaust system, particularly the exhaust manifold, catalytic converter, and muffler, also creates problems with heat transfer to other components of the vehicle, including under-hood components, the passenger compartment, and rear components such as the trunk and fuel tank. This problem is particularly acute when certain components, such as the fuel tank and the trunk, are formed of plastic materials like polypropylene. Shielding these components from the heat of the catalytic converter is therefore of great importance. Examples of shields developed to protect against damage caused by the high temperature components of a vehicle exhaust system include Atkinson et al., U.S. Pat. No. 5,385,790, and Moore, III, U.S. Pat. No. 5,590,524, the disclosures of which are incorporated herein by reference.
A shield for the components of an exhaust system must meet several criteria. It must be sufficiently efficient to provide adequate thermal shielding. It must withstand repeated exposure to high temperatures. It must be strong enough to resist damage from rocks and the like. It must not rattle or produce other noise during operation of the vehicle. It must accommodate differential thermal expansion of its hot inner part and its cooler outer part. It must be compact. Preferably, it should be lightweight, recyclable, and inexpensive to make, install, and maintain. Also preferably, it should promote rapid achievement of the catalytic converter's light off condition but should not allow, or at least should not exacerbate, overheating of the components of the exhaust system. At present, no shield meets these criteria adequately.